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tbi 305 or sbc 400?

8K views 34 replies 13 participants last post by  cj8scrambld 
#1 · (Edited)
Hey guys. Have a bit of a dilemma. I plan on doing an engine swap and this will be my first one I’ve ever really done. I’ve been doing forum searching and finding tons of different opinions and answers so I figured I’d ask real Chevy guys as you probably have the most experience.
I have my 87 with a tbi 305/th400/np208. I picked up a 2 bolt main sbc 400 from a 74 caprice that I'm looking to use for the swap. I have the HEI distributor for it and 4 bbl carb so far. Now as I’ve been reading and asking around, I’m getting mixed answers. I’ve been told the common, “no replacement for displacement” and that I’ll love the torque from the 400. but a local guy here said that he wouldn’t do it because he said the 87 with the sfa and tbi was a great combo and the tbi will sip on fuel compared to the 400 when offroad. Now for me, I’m not too worried about fuel (but I would obviously rather not spend tons of $$ on fuel) as it will be my weekend rig (I carpool to work everyday anyways). I just find he makes a good point. But again, it is a 305. If it was a tbi 350, I don’t think I would have even picked up the 400. I paid $300 for it so it was a good deal to me. This could very well be a stupid question, I’m just trying to get ideas of what my best bet is. Im not going to run over 35" tires and do a mix of trail/mountain riding with mud holes on the way.
What do you guys think? Should I go ahead with the swap? Any ideas or experience? I’m still learning and I’m on the Chevy forum here everyday trying to soak up any knowledge and ideas I can get. Any help/advice would be appreciated. Thanks!
 
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#3 · (Edited)
oh, im most definitely planning on kicking ass lol. im just at a major point in my build where i need to make this decision. i've also been doing some research on the tbi 400. but ive found kits online for about $2000 so that’s a little out of my price range for now. From what ive read as well, putting the 305 tbi or even a 350tbi on the 400 wont be enough for the bigger motor. Even if I have to go with 454 tbi, I'll do it, just wondering if anyone had experience doing this maybe? Still learning as I go, but I definitely want to get my hands dirty doing this. As well, just wondering if it makes sense to go from tbi to carb as carb is the old technology…im getting answers like that from some people.
 
#5 · (Edited)
i actually own and am using both those engines, what are the odds.

i got a 1990 chevy pick up truck with a tbi 305 in it and a 86 k30 with a 73 400 in it.

hands down the 400 as long as its not a piece of crap. the 305 is a good reliable smooth running engine, but that 400 is a torque monster. i had that thing in a 82 suburban with 38 inch tires and 373 gears and i pulled a car with it and you didnt even know it was there. the 305 in a stock truck pulling my s10 does the job but it sure isnt no speed demon, takes a while to get to 55 but it will hold the speed once you get there.

also theres the simplicity of the 400. just a couple wires going to the engine adn your down with it. getting that tbi to work might be a little bit more touble, but might be worth it.

it all kinda falls into your hands.
if the 305 is already in the truck and it runs fine then id probably leave it the hell alone. that 400 might be crap
 
#8 ·
Thank you abig84. good to hear from someone who has the same motor. i think im going to just throw the 400 in. its my dd but my drive to work is about a few miles so the most gas consumption i'll be worried about is on the trails. plus im starting to tear the 400 down as we speak so i think the peace of mind of knowing the motor has been freshened up and inspected will be a good clean slate to start with.

For the motor, im not looking to go all out as I can't really fund it as well as i have no clue what i'm doing. im just going to tear it down and just clean up all the crap on the motor and re-seal it. When i do this, i know to take a look at the timing chain as well as a good inspection of the block and look for scoring or other bad wear. the guy i bought it off said it read 48,000 kms. so even if its 148,000 kms, thats still not too bad. I also have a TH400 and NP208 that i would like to freshen up to put all 3 in at once. like i said, i would like the peace of mind on the trails knowing that all 3 components have been worked on. If im missing anything to look at on the 400 and if you guys have any rookie tips to do basic service on the TH400 and NP208, i'd greatly appreciate it. i've done the standard google searches but once again, im looking to see if anyone has any first hand experiences or hints/tips/secrets that theyve found out.

thanks again
 
#12 ·
THIS is what I am doing. I am using a rebuilt/bored BBC TBI unit from tbiparts.com along with 90# injectors run at a higher pressure. My 406 will be a monster in the TQ department and will definitely make my 6000# truck move.

Call tbiparts (spr performance) as I believe they were working on a mono-blade SBC TBI which could save you some $$ as you would already have a core.
 
#16 ·
So I've been talking to people at Harris Performance and this is what i was told:

Well stock 1974 caprice 400sbc made 200hp with a similar cam that what GM ran in the 180hp 350 TBI motor. You should be able to run that 200hp motor with a 350 TBI setup with a 15psi spring from www.tbiparts.com to increase that fuel pressure 15% over the stock 12psi to cover the extra 15% more power from the 400.
Only difference between a 305 TBI unit and a 350 TBI unit is the injectors. You will need the 5235206 350 truck injectors. The other people are advising based on modified 400ci motors. Stock they only made 200hp but most have modified or replaced the heads and put a decent cam in them where they can quickly make over 300hp and then you do need more air and fuel

A 305 distributor will bolt right in and the intake just use 3/8” bit and oblong the center 4 holes where the bolt can go in at different angle and it should work fine or use carb intake with an adapter. As for ECM the 1227747 computer should be fine. Now the 305 is 95ci difference and 25% difference in injectors will throw everything off too far. Using a 350 chip with 350 injectors running another 15% more fuel pressure on those injectors to get them to flow more than the computer is programmed for to cover that extra 15% worth of displacement. A performance chip would gain a little more power as well but that will work.


You guys have any input for that? What i'm getting is that i am basically replacing the manifold down. looks like i'll just need a tbi 350 unit, an ecm and a spring for the injectors. Am i missing anything?

This seems alot easier than some threads i've seen online...:confused:
 
#17 · (Edited)
Am i missing anything?
Yes, you are missing a hell of a lot.

First, a 350 TBI is roughly 205 HP.

Second, It isn't just about how much fuel you can flow. It's about how much you push within a given set of input parameters.

What you are describing is a recipe for an abortion. The ECM injects fuel according to a set of relatively simple tables, but they are set in stone. It looks for manifold pressure, throttle input, RPM, etc, and injects fuel according to those tables. It isn't enough to assume that it only needs X times the fuel stock. The tune has to match the engine, relatively precisely. I'm going to go out on a limb and guess that it will run, and in a steady state, it will work just fine. Drivability is going to be the issue. off idle will probably suffer, acceleration enrichment will require some tuning, etc.

That doesn't even begin to touch on timing. Do you really think the timing requirements for a 74 Caprice 400 are the same as a late model TBI 350. TBI engines don't require a whole lot of advance, on the other hand, your 400 probably will. Going to just advance that distributer? That will work for idle, but makes for a poor running engine overall. It'll need a different curve. Know how to re-curve a TBI setup. Yea, that's right. It's in the tune.

By all means, bolt it all together. It's pretty straightforward. At that point, you can sort out exactly what you have to do to get it to run right. Just don't expect it to work the way you seem to think it will.

YOU WILL HAVE TO TUNE IT TO THE ENGINE. It isn't about a performance chip. It's about a chip that will work with your engine.
 
#19 ·
ok, ok. thanks guys. i know it seemed to good/easy to be true. had my hopes up high lol. :(

oh well. now i at least have a clear direction im going in. My plan now is to buy a wiring harness for a carb setup. would like to buy one so i can get it fresh and set up properly. Off topic, but i have a few questions if you guys know the answers to, it would be great. apparently when i switch to carb from tbi, i'll need to get some sort of lockup kit? (i dont know exactly what its called). Also, I've looked at my Rochester 4 bbl carb. Woooooow. :eek:

For a noob, thats alot for me to grasp. i understand its older, simple technology but i have no idea how to set this up. guess i'll have to look into it more. what do you guys think about the 4 bbl rochester on the 400 for offroading? im going to hell of a time figuring it out

have a good one.
 
#20 ·
the quadrajet is probably one of the best carbs for offroading.

most problems come from someone "supertuning" it.

you already have the wiring you need. you just need to separate the stuff you dont need in the harness and run what you do need.

it really isnt too difficult. Im in the middle of rewiring a 68 chevy to use an HEI and a CS144 alternator on a 99 vortec motor. there are only half a dozen wires needed to run it.

starter wire
HEI positive wire
temp sending unit wire
Alternator charge wire to starter
Alternator field wire to instrument cluster or ignition switch. (2 possibilities, 2 wires).

you might have to include the oil pressure sending unit if yours is not a mechanical guage.
 
#24 · (Edited)
Thank you. that was probably the best explanation of it ive found so far.

OP, don't let all this dissuade you. It is more complicated than you initially guessed, but it isn't rocket science. It will require that you educate yourself a little.
Youre right. Thank you. Reading what you guys are writing is nice to see. i cant believe how much of a noob i am compared to you guys lol :eek:



I think i'll just carb the 400 for now and research into an eventual tbi for it. Its not my DD so its not a big deal if it guzzles gas for a bit. My problem right now is to get the qjet ready to go on the intake. i know its old technology, but im pretty confused with how to set up the vacuums, linkages, etc...

Guess Google will be my best friend for a while :D

Oh, and I almost forgot. What do I have to do about the torque converter lockup? Im seeing alot of mention of this (if I'm even getting the term right)
 
#23 ·
i never said it would run the best it could or that it wouldn't run better with some tuning, but with a smallish cam and stock tbi components with the fuel psi mods and injector work i think the 400 would run just fine. it'd be easy to argue the even a stock tbi engine and a stock tune isn't even "right" by you're use of the term. even in stock form the earlier injection didn't fuel the engine "right" as most ran pretty lean.

i didn't come here to argue or say my way is the only good way to do it. i don't claim to be the all knowing expert either. i just know that in my experiance with tbi and having swapped it onto a few different engines including a 383 a few warmed over 350's, 4.2 jeep motors and even a ford 351 in a boat. with a few simple mods they all ran pretty damn good. yes the ones that got custom chips ran better but they were all liveable on stock tunes.
 
#25 ·
the one time i actually took the time to figure out my gas mileage in the 400 i got 6.9 mpg. which wasnt to bad since that was when it was a 82 6089lb suburban with 373 gears and 38 inch tires. and a few hours of that was in 4low playing in the mud.

also it had the stock 2bbl on it. now its got a holley 750 on it so im sure it isnt any better
 
#27 ·
im to lazy to read back to find the exact quote and im going to bed after this post so ill tell the little bit i know :flipoff2:

if you are referring to the th400 transmission it has a electrical kick down control unlike the 700r4 or th350 which uses a cable. its basically a 12v signal sent to the transmission via a switch which is part of the gas pedal.

so it really isnt that hard to do. you just snag the bracket and switch from a junkyard and run some wire to the tab on the side of the transmission to the switch. i ran without it for years it just sucks if you are trying to pass.

if you dont know what the kick down gear is at all. then picture cursing at 25 mph in 3rd gear, you want to floor it to speed up, in 3rd gear the truck will be slow as hell. activating that kick down will throw the transmission into 2nd to get more torque and get you moving
 
#29 ·
yeah its actually part of the gas pedal so when the pedal is pushed down like halfway or so it just connects the circuit and the transmission will downshift.

if you reallllllllly wanted to be a cheapass you could just hook it to a toggle switch or button and push it as soon as you floor it. it would be kinda like a turbo boost lol. but yeah like i said you should be able to find them in the junkyard or maybe even on ebay.
 
#30 ·
Yea so i am actually running a tbi 400 sbc, has the 350 set up right now, runs ok at best but after talking to the boys at tbi parts and tbi chips i'm getting a chip set up for the motor, cam shaft with enough vacuum to support the tbi, a modified 54 mm 454 tbi unit that will use my 350 injectors and a better fuel pump.
 
#34 ·
Im officially installing the 400 this weekend. Im not going to bother with making it tbi just because of the headaches im sure I'll get being inexperienced. I'm going to use my current alternator bracket and rad hoses and rad (all good condition). Just a few last points before I do it that I'm looking to get cleared up:

1. Is it possible to disconnect power to the elec pump and run my fuel line to the mechanical pump? I'll just have to remove/disable the return line, correct?
2. Kickdown switch - I'm almost positive I'll be going the 12V switch route. Is it just needed between 2nd-3rd gears? Or is it required for 1st-2nd too? If so, I don’t understand how it works…
3. The 400 has no power steering pump on it. My current one looks like a bolt on to the 400. anyone ever have any problems with this? I couldn't imagine not having p/s in a truck while off road….
4. Anyone know how the vaccuum lines work for the 4 bbl qjet? I'm pretty stumped here. Some local guy (sketchy at best) said, "just block off the lines with a screw or something".

That’s all I can think of right now. I'll be heading up to the shop tonight to sort through all the wiring and have it ready for a drop on. Fingers crossed. My first swap ever.

Cheers!
 
#35 · (Edited)
Been a plan of mine for a while.....350tbi heads/intake/f.i. ontop of a 400 5.7 rodded shortblock. 9.6:1 KB #126 hypers running a marine/ramjet 350 roller (converted block of course). With the right tune it should be a beast. DO IT!

Edit: I would experiment with increased fuel press (which increases lbs/hr of the injectors) before going with bigger injectors. The tbi marine roller is the same grind as the ramjet 350. That grind worked well in the oem marine 350 so it is mild enough to not cause tuning headaches.
 
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